Reid ate 1/4 if pie, joe ate 1/3 of pie. how much was left after they both ate their pieces

Considering Reid ate 1/4 of the pie and Joe ate 1/3 of the pie, we have to bring this to their common denominator. In this case that is 12.

1/4 + 1/3 = ?

(1 * 3)/12 + (1*4)/12 =
3/12 + 4/12 = 
(3 + 4)/12 =
7/12

But this is the amount of the pie that both Reid and Joe ate. 

To calculate the remaining amount we have to imagine our pie was still whole (12/12)
and substract what they both ate (7/12):

12/12 - 7/12 = 5/12 

Only 5/12 of the pie was left, a little less than half of it :)